Gamma-band synchronization between neurons in the visual cortex is causal for effective information processing and behavior.

Successful behavior relies on the brain's ability to process selectively attended information while suppressing irrelevant information. Visual neurons show such functional flexibility by selectively responding to subsets of inputs representing attended objects while ignoring those conveying information about irrelevant objects. Several neuronal mechanisms have been proposed to explain this attention-dependent processing, yet none has been proven as a causal mechanism. One requires precise synchronization between spikes carrying relevant information and the gamma-oscillatory activity in receiving neurons. To investigate its causal relevance, we electrically evoked single volleys of spikes in area V2 of two male macaque monkeys performing a selective-attention task and recorded neuronal activity in downstream area V4. Strongly depending on the γ-phase, when these additional spikes arrived in V4, they impaired monkeys' performance and evoked a spiking response. This establishes the causal relevance of subtle changes in spike timing, specifically by phase synchronization, for neuronal mechanisms serving cognitive processes.